The present invention relates generally to applying currents and making electrical measurements on cylindrical members such as pipes in order to detect defects therein. More particularly, it relates to facilitating the engagement or attachment of multiple electrodes or contacts to the external surface of an object, such as a pipe, and to the use of ECCT (Electric Current Computed Tomography) technology to determine defects therein.
Numerous studies have been made as part of an overall effort to improve the performance of pipes and other such structures relative to damage from flaws in the structures which can lead to defects, such as cracking, in the walls thereof. One type of degradation which has received and which deserves special attention is stress corrosion cracking. Stress corrosion cracking occurs where the pipe is utilized to contain material having high temperature, high pressure steam, water, or other fluids.
An effective way to study defects, such as cracks, which may occur within a pipe wall, is to utilize electrical measurements of the pipe and to observe and record changes in the electrical potential and flow of current in the pipe both before and after a defect, such as cracking, has been initiated and as the defect enlarges.
In order to study the cracking phenomena within pipe walls and other objects, it is frequently desirable to make numerous measurements essentially around the entire circumference of the pipe and, if believed necessary, along the entire length of the pipe from one contact point to another spaced axially along the pipe length.
When the application of a large number of current patterns and the measurement of a large numbers of voltages on a pipe can be accomplished, the data which is generated from such measurements can be analyzed by a number of techniques including Electric Current Computed Tomography or ECCT. With the aid of the numerous surface contacts attached to the pipe, ECCT provides a determination of the pipe condition as derived from such electrical measurements.
Accordingly, a problem with the use of the ECCT technology in specific, and any crack monitoring technique in general, is that a large number of electrical contacts must be precisely applied to the pipe surface in order to permit the application of current, the measurement of voltage and other critical data that is collected. Commonly owned U.S. Pat. No. 4,982,154, which is expressly incorporated herein by reference, teaches one method and application for rapidly attaching to and removing a large number of electrical contact points from a cylindrical surface, such as a pipe. Once a fixture having a plurality of electrical contact points is attached, pneumatic activation was used in the commonly owned patent, to provide the pressure to achieve and sustain contact between individual contact points and the pipe surface.
With regard to making potential measurements for ECCT technology in specific and electrical potential crack monitoring method in general of piping, a reliable electrical contact for each of the many contact points on a pipe surface is required. For contact to be effective electrically, it must be one which does not change over time (i.e., firm and difficult to move during the testing). Further, it has been found to be most effective if the contacts are essentially evenly spaced both along the pipe surface and about the pipe circumference in order to provide the proper geometry required to accumulate the data needed for an ECCT analysis and study. Once the regular geometric array of electrical probes is firmly attached to the surface of the object or pipe, different levels of current and voltage can be applied to and/or sensed therefrom.
The commonly owned patent did not specifically address variations in the surface contact resistance that would occur between different probes. If groups of probes are tied together electrically (to create a single electrode), current preferentially flow through the pin/surface interface that provides the least resistance. This condition would make mathematical modeling of the pin/structure interaction impossible because the details of the surface phenomena would be required to develop an accurate model since a predictable, repeatable distribution of current among the pins is a requirement. Thus, if there is resistance differences between the pins, problems arise making mathematical modeling of the pin/structure interaction impossible.
While the apparatus and method of the aforementioned commonly owned patent solved some problems related to pipe crack analysis, several new problems specific to the new apparatus and method surfaced which made the utilization thereof impractical. First, since gas leakage around the pins appears to be unavoidable, a constant supply of pressurized air was required for the fixture. Such a constant supply required either a local pump or a pressurized gas line. Since the apparatus and method of the commonly owned patent would most likely be used in situations where such an extended complicated gas supply system would create significant inconvenience, such as piping in a nuclear reactor which is contained within the pressure vessel, it was determined that the significant inconvenience and increased cost made the invention of the commonly owned patent impractical. Second, failure of one pin seal disabled the entire fixture, since no significant pressure would be created within the pneumatic chamber. Since the apparatus and method of utilizing the pneumatic seal would be performed in locations having at best limited access, there would be no easy, convenient way to correct the disabled seal problem. The failure of any individual seal would result in the failure of the entire device causing it to have to be removed from the pipe in order to effectuate repair.
Notwithstanding, the presentations of the art discussed above, there continues to be a need for an improved method and system for detecting defects in pipes or other structures. Such a method and system would provide for the rapid and reliable attaching of one or more fixtures having a plurality of electrical elements positioned therein for use with a mathematical model in the analyzation of a pipe or other object for defects; would have individual tension means, such as a spring, for assisting the electrical elements to make individual electric contact with the pipe or other object; and would provide for uniform current distribution over the surface of the pipe or object being maintained.